What is Swimmers’ Itch?

Swimmers' itch is caused by flat worms (larva schistosome parasites) that penetrate the skin. The microscopic-sized parasites die shortly after penetrating the skin, but can cause allergic reactions in some people.

What are the symptoms?

A reddened spot appears within a few hours after the parasite enters the skin. The spot may grow in size - especially if scratched - and the skin surface swells, sometimes appearing as red welts. The irritated spot reaches its maximum size after about 24 hours; the itching may continue for several days. The symptoms should disappear after a week.

How do you get swimmers’ itch?

The parasites are found in shallow water near the shoreline. They look for mammals or birds to attach to. The parasites penetrate the outer layer of skin within several minutes after making contact.

When can you get swimmers’ itch?

The first outbreaks of swimmers' itch usually occur in late May or early June. Lakes in the northern half of Illinois usually have a swimmers' itch season of about two weeks.  Lakes in the southern half of the state may have a season lasting one month. However, there's no way to predict how long an outbreak may last. In some lakes it may last an entire summer.

Where does the parasite come from?

The schistosome parasite has a complex life cycle that uses aquatic birds or rodents and snails as hosts at different times during its maturation. Common grackles, red-winged blackbirds, ducks, geese, swans, muskrat, and moles have been found to carry the parasite.

The birds and rodents carry the adult schistosome parasite, a flat worm, in their blood vessels. Female adults lay their eggs in the host's blood vessels. The eggs move into the intestine and are expelled when the host defecates. The eggs hatch when they're expelled into water and grow into free-swimming larva (maracidia) that take up residence in snails. Using the snails as hosts, these larva develop into cercaria larva. It's these cercaria that cause swimmers' itch when they look for a new host to complete the parasite's life cycle.

Should a doctor be consulted?

In severe cases of swimmers' itch the infected person may have a fever, feel nauseated, and have difficulty sleeping. Consult a physician if you're not sure what caused the rash, if the symptoms intensify or persist longer than one week, or if the symptoms cause undue discomfort. A physician can prescribe medication to reduce itching and topical creams to reduce swelling.

What preventive actions can lake property owners take?

A couple things will help:

• Don't encourIage birds to stay near a swimming area by feeding them.
• Don't riprap the lakeshore if possible - the rocks provide habitat for snails.

Can swimmers lessen their chances of getting swimmers’ itch?

Only about one third of the people who contact the schistosome parasite develop swimmers' itch.

Children appear to be more susceptible since they tend to swim more often and swim in shallow water where, the cercaria larva tend to accumulate.

There are several measures that reduce the risk of contracting swimmers' itch:

• Towel down immediately upon leaving the water.
• Swim in water away from the shore.
• Avoid swimming immediately after an onshore wind if the lake is having problems with swimmers' itch.
• Avoid areas where snails have accumulated.

Lake Management Program Wisconsin Department of Natural Resources. PUBL-WR-170 87 REV November 1987.
For more information contact Frank Koshere, WDNR Servicce Center, 715-392-0807, E-mail: koshef@dnr.state.wi.us

Last updated 14 May 2001